1. Field of the Invention
The present invention relates to a photographic lens system, and more particularly to a focal length changeable lens system in which the foxal length of the photographic lens system can be changed by inserting and retracting a rear conversion lens group on the image side of a master lens group.
2. Description of the Prior Art
Conventionally, it has been considered to apply a lens element made of a synthetic resin material to an element in a lens system for decreasing the manufacturing cost of the lens system. However, such a lens system, including at least one lens element made of a synthetic resin material, has a drawback that the position of image plane of the lens system changes due to the fact that the back focal length of the lens system changes with a change in temperature. Such a drawback also occurs in a focal length changeable lens system in which the focal length of the photographic lens system can be changed by inserting and retracting a rear conversion lens group on the image side of a master lens group. In such focal length changeable lens system, the position of image plane changes seriously in the long focal length condition when the rear conversion lens group is inserted behind the master lens group.
In a focal length changeable lens system in which the master lens group includes at least one lens element made of a synthetic resin material, and in which the rear conversion lens group has no lens element made of a synthetic resin material, the change of back focal length .DELTA.L.sub.BT, in the long focal length condition in which the rear conversion lens group is inserted behind the master lens group, is represented as follows: EQU .DELTA.L.sub.BT =.DELTA.L.sub.B .times.M.sup.2 (A)
wherein; .DELTA.L.sub.B represents the change of the back focal length of the master lens group with the change in temperature, that is, the change of the back focal length of the lens system in a short focal length condition in which the rear conversion lens group is retracted from the optical axis of the lens system, and M represents the magnification of the rear conversion lens group. Namely, even if the change of the back focal length in the short focal length condition is controlled within a permissible range, the change of the back focal length in the long focal length condition may violate the permissible range. Such a change may be significant when the magnification of the rear conversion lens group is considerably large. In the long focal length condition, errors in manufacturing the master lens group, errors in manufacturing the rear conversion lens group, and errors in position of the rear conversion lens group relative to the master lens group influence the change of the back focal length of the whole lens system. Therefore, the back focal length of the whole lens system changes significantly in the long focal length condition with a change in temperature.
In the above-identified focal length changeable lens system, another approach has been considered for decreasing the change of the back focal length of the whole lens system due to the change in temperature. Such an approach is to design the master lens group so that the change of back focal length in the master lens group is controlled to almost zero and to design the rear conversion lens group so that the change of the back focal length in the rear conversion lens group is also controlled to almost zero, respectively. However, if such an approach is applied to the design of a lens system, both of the constructions of the master lens group and that of the rear conversion lens group become complex, because of an increase in the number of lens elements in both of the lens groups.
Further another approach is considered for decreasing the change of the back focal length of the whole lens system due to the change in temperature. Such an approach is to compensate the change of back focal length of the whole lens system in the camera body side. However, if such an approach is applied to design a camera body, the construction of the camera body becomes more complex.